I guess the question is what is the pricing difference between K class (lowest mileage-upgradeable Y fare) and the cheapest Y+ class?

I agree it seems a little strange, because if the miles required for an upgrade is the same then pax desiring an upgrade to J will follow the below logic, which may have some unintended consequences for CX:
a.) IF price of K class and cheapest Y+ ticket are the same, then purchase Y+ (may as well buy Y+ because you get 110% base miles, in the off chance the upgrade doesn't come through you fly in more comfort than Y-, and you can still upgrade), or
b.) IF the price of K class is cheaper, then you may as well buy the Y- ticket and upgrade (assuming the price difference more than compensates the fewer miles you'll get).